Method for selectively charging and discharging a steering accumulator

ABSTRACT

A system and method for selectively charging and discharging a steering accumulator are provided. One vehicle steering system includes a hydraulic pump configured provide a source of pressurized hydraulic fluid for use in steering the vehicle. The steering system also includes an accumulator configured to hold pressurized hydraulic fluid from the pump for use in steering the vehicle in case of need. The steering system includes a charging circuit configured to selectively allow the hydraulic fluid to flow from the hydraulic pump to the accumulator based on a fluid pressure applied by a first pilot and a second pilot. The steering system includes a charging bypass circuit configured to allow the hydraulic fluid to flow to a steering control unit and to bypass the charging circuit. The hydraulic pump supplies fluid to the charging circuit and the bypass circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/205,789 filed Aug. 9, 2011, by Jerry Lee Brinkley and AdamRobert Rusciolelli, “System and Method For Selectively Charging AndDischarging A Steering Accumulator”, which is incorporated by referenceherein for all purposes in its entirety.

BACKGROUND

The invention relates generally to hydraulic steering systems and, moreparticularly, to a system and method for selectively charging anddischarging a steering accumulator.

Power steering systems allow an operator to turn a steering wheel of amotorized vehicle with greater torque than in systems without powersteering. Thus, a vehicle equipped with power steering is easier tomaneuver than the same type of vehicle without power steering. Incertain vehicles, such as certain agricultural vehicles, off-roadvehicles, work vehicles, and the like, turning a steering wheel withoutpower steering may be very difficult. Many vehicle power steeringsystems are hydraulically based, particularly in such large vehicleapplications. In hydraulically based power steering systems, a hydraulicpump generally pumps hydraulic fluid to be used by the power steeringsystem. As may be appreciated, certain faults may occur in powersteering systems that cause the systems to not function properly. Forexample, the hydraulic pump may stop functioning, a control unit may notwork properly, an engine failure may occur, or hoses carrying hydraulicfluid may leak.

To overcome certain power steering problems, power steering systems mayuse an accumulator to store a supply of pressurized hydraulic fluid thatcan be used as a backup if pressure from the main hydraulic fluid supplydrops too low. In such systems, the accumulator may be charged while thehydraulic system is functioning properly. If a problem is detected, thepower steering system switches from using the main hydraulic fluidsupply to the backup hydraulic fluid supply from the accumulator. Thisallows a vehicle operator to maneuver the vehicle for a short period oftime, such as for moving the vehicle from a location on a road to aroadside or parked location. However, known power steering systemsimplement accumulator charging and/or discharging in an inefficientmanner. Accordingly, there exists a need for steering accumulatorcharging systems that efficiently charge and/or discharge the steeringaccumulator.

SUMMARY

In one embodiment, a vehicle steering system includes a hydraulic pumpconfigured to provide a source of pressurized hydraulic fluid for use insteering the vehicle. The steering system also includes an accumulatorconfigured to hold pressurized hydraulic fluid from the pump for use insteering the vehicle in case of need. The steering system includes acharging circuit having a charging valve and a hysteresis valve. Thecharging valve has a first pilot and a second pilot. The charging valveis configured to selectively allow the hydraulic fluid to flow from thehydraulic pump to the accumulator based on a first fluid pressureapplied by the first pilot and a second fluid pressure applied by thesecond pilot. The hysteresis valve is configured to selectively allowthe hydraulic fluid to flow from the charging valve to the first pilotof the charging valve. The steering system includes a charging bypasscircuit configured to allow the hydraulic fluid to flow from thehydraulic pump to a steering control unit and to bypass the accumulatorcharging circuit. The hydraulic pump supplies fluid to the accumulatorcharging circuit and the charging bypass circuit.

In another embodiment, a method for charging an accumulator of a vehiclesteering system is provided. The method includes opening a chargingvalve to allow hydraulic fluid to flow from a hydraulic pump to anaccumulator. The method also includes closing a hysteresis valve whenthe hydraulic fluid in the accumulator reaches a first fluid pressure.Before the hysteresis valve is closed, hydraulic fluid flowing throughthe hysteresis valve applies pressure via a first pilot of the chargingvalve. The method includes closing the charging valve after thehysteresis valve is closed. The charging valve is closed when thecombined pressure applied by the first pilot and a spring drops belowpressure applied by a second pilot of the charging valve. The methodalso includes opening the charging valve to allow hydraulic fluid toflow from the hydraulic pump to the accumulator when the pressureapplied by the second pilot of the charging valve decreases below thecombined pressure applied by the first pilot and the spring.

In another embodiment, a vehicle steering system includes an accumulatorcharging valve configured to allow hydraulic fluid to flow to anaccumulator when the combined force applied to the charging valve by aspring and a first pilot is greater than the force applied to thecharging valve by a second pilot.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a perspective view of an embodiment of an agricultural vehiclethat may employ a vehicle steering system with an accumulator chargingcircuit in accordance with the present disclosure;

FIG. 2 is a block diagram of an embodiment of a vehicle steering systemwith an accumulator charging circuit in accordance with the disclosure;

FIG. 3 is a schematic diagram of an embodiment of an accumulatorcharging circuit generally conforming to the embodiment of FIG. 2; and

FIG. 4 is a flow chart of an embodiment of a method for charging anaccumulator of a vehicle steering system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 illustrates an exemplary agricultural vehicle 10 that may employa vehicle steering system with an accumulator charging circuit. Incertain embodiments, the agricultural vehicle 10 may be any type oftractor, off-road vehicle, work vehicle, or any other suitable vehiclethat utilizes a power steering system with a need for a hydraulicbackup. The vehicle 10 illustrated has a body 12 that will typicallyhouse an engine, transmission, and power train (not separately shown).Further, the agricultural vehicle 10 has a cabin 14 where an operatormay sit or stand to operate the vehicle 10. The vehicle 10 has two frontwheels 16 and two rear wheels 18 (only one is shown) that rotate to movethe vehicle 10. As may be appreciated, the vehicle 10 is maneuveredusing a steering wheel 20 that causes the wheels 16 to turn. The wheels16 are coupled together by an axle 22 so that both wheels 16 may berotated together. The agricultural vehicle 10 may include a vehiclesteering system with an accumulator charging circuit. The vehiclesteering system may charge the accumulator during normal operation sothe accumulator may be used as a backup source of hydraulic fluid if thestandard vehicle steering system does not function properly. It shouldbe noted that, depending upon the vehicle design, the steering systemmay control the movement of directional wheels, or other structures maybe controlled for steering, such as articulation joints between portionsof the vehicle frame.

FIG. 2 is a block diagram of a vehicle steering system 34 with anaccumulator charging circuit. The system 34 includes a hydraulic fluidreservoir 36, or, in other embodiments, some other type of hydraulicfluid source. Hydraulic fluid is pumped from the reservoir 36 throughthe system 34 by the hydraulic pump 38. The hydraulic fluid flows fromthe pump 38 to upstream valving 40. For example, the upstream valving 40may include various valves, such as relief valves, check valves,directional control valves, pressure regulating valves, and so forth. Asillustrated, the hydraulic fluid flows from the upstream valving 40 toan accumulator charging circuit 42 where an accumulator is charged foruse in the steering system 34. The hydraulic fluid may bypass theaccumulator charging circuit 42 by flowing through the bypass circuit44. When the hydraulic fluid flows through the bypass circuit 44, thefluid flows to a steering control unit 46 without passing through theaccumulator charging circuit 42.

In contrast, when the hydraulic fluid flows to the accumulator chargingcircuit 42, it does not flow to the steering control unit 46 until aloss of hydraulic fluid pressure is detected in the bypass circuit 44.The steering control unit 46 uses the hydraulic fluid to provide powersteering to the vehicle 10. It should be noted that the system 34illustrated uses only one pump 38 to supply hydraulic fluid to both theaccumulator charging circuit 42 and to the bypass circuit 44. Therefore,the system 34 is designed to function in an efficient manner byutilizing a single pump 38 for multiple functions (e.g., the accumulatorcharging circuit 42 and the steering control unit 46). Further, thebypass circuit 44 allows a majority, or during extended periods, all ofthe hydraulic fluid to bypass the accumulator charging circuit 42. Assuch, the components of the accumulator charging circuit 42 are exposedto lower wear and tear than systems that have the hydraulic fluid forthe steering control unit 46 generally flow through the accumulatorcharging circuit 42.

FIG. 3 is a schematic diagram illustrating an embodiment of theaccumulator charging circuit 42. As previously discussed, hydraulicfluid flows from the upstream valving 40 toward the accumulator chargingcircuit 42. In particular, the hydraulic fluid flows to a connection 52that connects a flow path 54 to the bypass circuit 44. The flow path 54enables hydraulic fluid to flow from the connection 52 to a chargingvalve 56 of the accumulator charging circuit 42. The charging valve 56has two positions, an open position 58 that allows hydraulic fluid toflow through the charging valve 56, and a closed position 60 thatinhibits hydraulic fluid from flowing through the charging valve 56. Asmay be appreciated, the charging valve 56 alternates between the openand closed positions 58 and 60 during operation of the accumulatorcharging circuit 42.

The charging valve 56 is moved to the open position 58 by pressureexerted on the valve 56 by a biasing spring 62 and pressure in a firstpilot 64. Conversely, the charging valve 56 is moved to the closedposition 60 by pressure exerted on the valve 56 by pressure in a secondpilot 66. Initially, before the hydraulic fluid flows to the accumulatorcharging circuit 42, the charging valve 56 will be in the open position58 based on the pressure exerted by the spring 62. The charging valve 56then moves between the open and closed positions 58 and 60 as the firstand second pilots 64 and 66 apply and remove pressure. For example, whenthe combined pressures (or forces) exerted by the spring 62 and thefirst pilot 64 are greater than the pressure (or force) exerted by thesecond pilot 66, the charging valve 56 will be in the open position 58.In contrast, when the combined pressures (or forces) exerted by thespring 62 and the first pilot 64 are less than the pressure exerted bythe second pilot 66, the charging valve 56 will be in the closedposition 60.

When the charging valve 56 is in the open position 58, the hydraulicfluid flows through the charging valve 56 toward a connection 68. Anorifice 70 (or restrictor) slows the flow rate of the hydraulic fluidfrom the charging valve 56 to the connection 68. Therefore, a greateramount of the hydraulic fluid will flow through the bypass circuit 44than will flow through the charging valve 56. From the connection 68,the hydraulic fluid flows to flow paths 72 and 74. Specifically, thehydraulic fluid flows through the flow path 72 to a hysteresis valve 76.The hysteresis valve 76 also has two positions, an open position 78 thatallows hydraulic fluid to flow through the hysteresis valve 76, and aclosed position 80 that inhibits hydraulic fluid from flowing throughthe hysteresis valve 76. The hysteresis valve 76 alternates between theopen and closed positions 78 and 80 during operation of the accumulatorcharging circuit 42.

The hysteresis valve 76 is moved to the open position 78 by pressure (orforce) exerted on the valve 76 by a spring 82. Conversely, thehysteresis valve 76 is moved to the closed position 80 by pressure (orforce) exerted on the valve 76 by a third pilot 84. Initially, beforethe hydraulic fluid flows to the accumulator charging circuit 42, thehysteresis valve 76 will be in the open position 78 based on thepressure (or force) exerted by the spring 82. The hysteresis valve 76then moves between the open and closed positions 78 and 80 as the thirdpilot 84 applies and removes pressure. For example, when the pressureexerted by the spring 82 is greater than the pressure exerted by thethird pilot 84, the hysteresis valve 76 will be in the open position 78.In contrast, when the pressure exerted by the spring 82 is less than thepressure exerted by the third pilot 84, the hysteresis valve 76 will bein the closed position 80. The hysteresis valve 76 includes a drain port86 that allows hydraulic fluid to flow from the hysteresis valve 76 tothe reservoir 36 when leakage in the valve 76 occurs. The drain port 86allows the hydraulic fluid to drain so that additional pressure will notbe applied to the valve 76 in conjunction with the spring 82.

When the hysteresis valve 76 is in the open position 78, the hydraulicfluid flows through the hysteresis valve 76 toward a connection 88. Fromconnection 88, the hydraulic fluid flows to the first pilot 64 to applyfluid pressure on the charging valve 56. The hydraulic fluid also flowsthrough a spring loaded ball check valve 90. The check valve 90 allowsthe hydraulic fluid to flow in one direction from the hysteresis valve76 to a load sense unit 94 (i.e., the hydraulic fluid is inhibited fromflowing from the load sense unit 94 to the hysteresis valve 76). Theload sense unit 94 monitors the load on the pump 38 to help maintain ahigh pressure in the accumulator charging circuit 42.

Returning to the connection 68, the hydraulic fluid also flows throughthe flow path 74 to another spring loaded ball check valve 98. Like thecheck valve 90, the check valve 98 allows the hydraulic fluid to flow inone direction. Continuing on, the hydraulic fluid flows from the checkvalve 98 to a connection 102. From the connection 102, the hydraulicfluid flows to flow paths 104 and 106 and to the second pilot 66. Inparticular, the hydraulic fluid flows through the flow path 104 to aconnection 108. The connection 108 splits the hydraulic fluid flow intotwo more flow paths 110 and 112.

Specifically, the hydraulic fluid that flows through the flow path 110fills an accumulator 114. Although only one accumulator 114 isillustrated, certain embodiments may include more than one accumulator114, such as embodiments that use a series of accumulators. Further, thesize of the accumulator may vary between embodiments. The hydraulicfluid flows to the accumulator 114 until the accumulator 114 is charged.In some embodiments, the accumulator 114 is charged when the accumulator114 is full of hydraulic fluid and when the hydraulic fluid pressure inthe accumulator 114 passes a minimum pressure.

The hydraulic fluid also flows from the connection 108 through the flowpath 112 to a discharge valve 116. The discharge valve 116 has twopositions, an open position 118 that allows hydraulic fluid to flowthrough the discharge valve 116, and a closed position 120. In theclosed position, a check valve symbolizes that the valve 116 is in a lowleak shut position. The discharge valve 116 alternates between the openand closed positions 118 and 120 during operation of the accumulatorcharging circuit 42.

The discharge valve 116 is moved to the open position 118 by pressureexerted on the valve 116 by a spring 122. Conversely, the dischargevalve 116 is moved to the closed position 120 by pressure exerted on thevalve 116 by a solenoid 124. Initially, before the hydraulic fluid flowsto the accumulator charging circuit 42, the discharge valve 116 will bein the open position 118 based on the pressure exerted by the spring122. The discharge valve 116 then moves between the open and closedpositions 118 and 120 as the solenoid 124 applies and removes pressure.For example, when the pressure exerted by the spring 122 is greater thanthe pressure exerted by the solenoid 124, the discharge valve 116 willbe in the open position 118. In contrast, when the pressure exerted bythe spring 122 is less than the pressure exerted by the solenoid 124,the discharge valve 116 will be in the closed position 120. When thedischarge valve 116 is in the open position 118, the hydraulic fluid mayflow through the discharge valve 116 to flow path 126 and then bedirected toward the reservoir 36. As may be appreciated, the solenoid124 may be energized using various methods. For example, the solenoid124 may be electrically connected so it is energized when the vehicle 10key is turned on, or the solenoid 124 may be energized when the steeringcontrol unit 46 is initialized.

Returning to the connection 102, the hydraulic fluid also flows throughthe flow path 106 to a backup steering valve 128. The backup steeringvalve 128 also has two positions, an open position 130 that allowshydraulic fluid to flow through the backup steering valve 128, and aclosed position 132. In the closed position, a check valve symbolizesthat the valve 128 is in a low leak shut position. The backup steeringvalve 128 alternates between the open and closed positions 130 and 132during operation of the accumulator charging circuit 42.

The backup steering valve 128 is moved to the open position 130 bypressure exerted on the valve 128 by a spring 134. Conversely, thebackup steering valve 128 is moved to the closed position 132 bypressure exerted on the valve 116 by a fourth pilot 136 (i.e., steeringhydraulic fluid pressure). Initially, before the hydraulic fluid flowsto the bypass circuit 44, the backup steering valve 128 is in the openposition 130 based on the pressure exerted by the spring 134. The backupsteering valve 128 then moves between the open and closed positions 130and 132 as the fourth pilot 136 applies and removes pressure. Forexample, when the pressure exerted by the spring 134 is greater than thepressure exerted by the fourth pilot 136, the backup steering valve 128will be in the open position 130. In contrast, when the pressure exertedby the spring 134 is less than the pressure exerted by the fourth pilot136, the backup steering valve 128 will be in the closed position 132.The backup steering valve 128 includes a drain port 140 that allowshydraulic fluid to flow from the backup steering valve 128 to thereservoir 36 when leakage in the valve 128 occurs. The drain port 140allows the hydraulic fluid to drain so that additional pressure will notbe applied to the valve 128 in conjunction with the spring 134.

When the backup steering valve 128 is in the open position 130, thehydraulic fluid flows through the backup steering valve 128 to anotherspring loaded ball check valve 142. Like the check valve 98, the checkvalve 142 allows the hydraulic fluid to flow in one direction. Thehydraulic fluid flows from the check valve 142, through an orifice 146(or restrictor) to a connection 148. The orifice 146 slows the flow rateof the hydraulic fluid from the backup steering valve 128 to theconnection 148. In certain embodiments, the restricted flow caused bythe orifice 146 allows a smaller accumulator 114 to be used in theaccumulator charging circuit 42 than would otherwise be possible. Inother embodiments, the orifice 146 limits the flow of hydraulic fluidfor the steering system 34 to meet certain regulations orspecifications.

Returning to connection 52, the hydraulic fluid flows from theconnection 52, through another spring loaded ball check valve 150 to theconnection 148. Like the check valve 142, the check valve 150 allows thehydraulic fluid to flow in one direction. At the connection 148, thefluid flow paths from the accumulator charging circuit 42 and the bypasscircuit 44 are combined so there is one flow path 154 from theconnection 148 to the steering control unit 46. The hydraulic fluidflows through the flow path 154 to the steering control unit 46.

The accumulator charging circuit 42 may operate according to thefollowing description. An operator may turn on the agricultural vehicle10. When the vehicle 10 is turned on, the solenoid 124 may be energizedand cause the discharge valve 116 to move to the closed position 120 asthe energized solenoid 124 applies a greater pressure than the spring122. With the discharge valve 116 in the closed position 120, hydraulicfluid is inhibited from exiting the accumulator 114 through the flowpath 126. With the vehicle 10 on, the hydraulic pump 38 may pumphydraulic fluid through the upstream valving 40 to the connection 52. Inparallel, the hydraulic fluid flows through the bypass circuit 44 andthe accumulator charging circuit 42. The hydraulic fluid flows throughthe bypass circuit 44 to the steering control unit 46 to operate thesteering system 34. The hydraulic fluid also flows through the fourthpilot 136 to begin applying fluid pressure on the backup steering valve128. When the pressure applied by the fourth pilot 136 (i.e., steeringhydraulic fluid pressure) is greater than the pressure applied by thespring 134, the backup steering valve 128 will move to the closedposition 132.

The hydraulic fluid that flows into the accumulator charging circuit 42flows concurrently to the accumulator 114 and to the hysteresis valve76. Further, until the backup steering valve 128 closes, some hydraulicfluid may flow through the backup steering valve 128 to the steeringcontrol unit 46. However, the hydraulic fluid will tend to flow throughless restrictive flow paths than the path that includes the orifice 146(e.g., the hydraulic fluid will tend to flow to the accumulator 114 orto the load sense unit 94). As hydraulic fluid flows through theaccumulator charging circuit 42, the accumulator 114 will fill up andbegin to be charged with pressurized hydraulic fluid. Further, thefirst, second, and third pilots 64, 66, and 84 will apply pressure tothe charging and hysteresis valves 56 and 76. As the pressure applied bythe second pilot 66 increases, the pressure applied by the first pilot64 will also increase. Therefore, the charging valve 56 will continue toremain open as long as the pressure applied by the spring 62 and thefirst pilot 64 are greater than the pressure applied by the second pilot66. In other words, during charging, the forces applied by the firstpilot 64 and the second pilot 66 generally remain equal and cancel eachother out with a net force of zero. Therefore, the spring 62 holds thevalve 56 open as long as the forces applied by the first pilot 64 andthe second pilot 66 are substantially the same.

As the pressure of the hydraulic fluid in the accumulator chargingcircuit 42 increases, the pressure applied by the third pilot 84 to thehysteresis valve 76 will eventually become greater than the pressureapplied by the spring 82 to the hysteresis valve 76. When this occurs,the hysteresis valve 76 will transition to the closed position 80. Inthe closed position, the flow of hydraulic fluid to the first pilot 64will stop and the pressure applied to the charging valve 56 by the firstpilot 64 will begin to decrease as the first pilot 64 drains to the loadsense unit 94.

When the combined pressure applied by the first pilot 64 and the spring62 decreases below the pressure applied by the second pilot 66, thecharging valve 56 will transition to the closed position 60. At thispoint, the accumulator 114 will be fully charged. However, when thecharging valve 56 is closed, the pressure applied by the third pilot 84to the hysteresis valve 76 will begin to decrease as hydraulic fluidexits the relief port 88. Further, the charged pressure of theaccumulator 114 will slowly decrease. After the pressure applied by thethird pilot 84 to the hysteresis valve 76 decreases below the pressureapplied by the spring 82 (e.g., by hydraulic fluid from the third pilot84 “leaking” through the hysteresis valve 76 into the reservoir 36), thehysteresis valve 76 will transition to the open position 78.

With the hysteresis valve 76 in the open position 78 (e.g., thehydraulic valve is reset), the hydraulic fluid will be able to flowthrough the first pilot 64. However, because the charging valve 56 is inthe closed position 60, there will be minimal hydraulic fluid flowing tothe first pilot 64. After the pressure applied by the first pilot 64 andthe spring 62 increases above the pressure applied by the second pilot66, the charging valve 56 will transition to the open position 58 toagain charge the accumulator 114. It should be noted that the pressureapplied by the first pilot 64 and the spring 62 to the charging valve 56will primarily be spring 62 pressure when the charging valve 56 is inthe closed position 60. Therefore, the charging valve 56 will remain inthe closed position 60 until the pressure applied by the second pilot 66is less than the pressure applied by the spring 62. The pressure appliedby the second pilot 66 decreases as the pressure in the accumulator 114decreases. The process of closing and opening the charging valve 56 andthe hysteresis valve 76 as previously described continue to repeatduring normal operation of the accumulator charging circuit 42. Thus,the charging circuit 42 keeps the charge on the accumulator 114 betweenan upper limit (e.g., maximum or cut-out pressure) and a lower limit(e.g., minimum or cut-in pressure). For example, the maximum pressureoccurs when the charging valve 56 transitions to the closed position 60and the minimum pressure occurs when the charging valve 56 transitionsto the open position 58. Therefore, the charging of the accumulator 114is accomplished using a hysteresis based input. In other words, theoperation of the charging valve 56 is hysteresis based.

When the accumulator 114 is charged, the accumulator 114 may be used toprovide the hydraulic fluid to the steering control unit 46 for alimited time. During operation, if the pressure applied by the fourthpilot 136 (i.e., steering hydraulic fluid pressure) decreases below thepressure applied by the spring 134, the backup steering valve 128 willtransition from the closed position 132 to the open position 130. Thiswill allow the hydraulic fluid to flow from the accumulator 114, throughthe check valve 142, through the orifice 146, and to the steeringcontrol unit 46. Further, if such a condition occurs, a sensor mayprovide feedback to the operator that there is a loss of power steeringpressure. As may be appreciated, the operator may also notice the lossof power steering pressure based on the difficulty of turning thesteering wheel 20.

In conditions where the accumulator 114 is not used to provide thehydraulic fluid to the steering control unit 46, the accumulator 114 isfully charged when operation of the vehicle 10 is complete. In suchconditions, the vehicle 10 operator will shut off the key and thedischarge valve 116 will transition to the open position 118. Theaccumulator 114 will then drain the hydraulic fluid it holds into thereservoir 36.

The valves 56, 76, and 128, as illustrated, are all hydro-mechanicalvalves. In other embodiments, some or all of the valves 56, 76, and 128may be electro-hydraulic valves. In addition, the check valves 90, 98,142, and 150 are all spring loaded ball check valves. In certainembodiments, the check valves 90, 98, 142, and 150 may be any type ofunidirectional valve. Further, it may be appreciated that the steeringsystem 34 may use the output from the accumulator charging circuit 42 inconjunction with a dual gerotor steering system.

FIG. 4 is a flow chart illustrating a method 170 for charging anaccumulator 114 of a vehicle steering system. At step 172, the solenoid124 is energized to close the discharge valve 116 so that the hydraulicfluid will be stored in the accumulator 114. Then, at step 174, thecharge valve 56 is opened to allow hydraulic fluid to flow into theaccumulator charging circuit 42 to charge the accumulator 114. Next, atstep 176, the accumulator 114 is filled with the hydraulic fluid. Atstep 178, the accumulator charging circuit 42 determines whether theaccumulator 114 is fully charged. If the accumulator 114 is not fullycharged, the charging circuit 42 continues to charge the accumulator114.

However, if the accumulator 114 is fully charged, at step 180, thehysteresis valve 76 is closed. After the hysteresis valve 76 is closed,the charging circuit 42 determines whether the pressure applied by thesecond pilot 66 is greater than the combined pressure applied by thespring 62 and the first pilot 64, per step 182. If the pressure appliedby the second pilot 66 is not greater than the combined pressure appliedby the spring 62 and the first pilot 64, the charging circuit 42continues to monitor the pressures until such a condition occurs. If thepressure applied by the second pilot 66 is greater than the combinedpressure applied by the spring 62 and the first pilot 64, at step 184,the charging circuit 42 closes the charging valve 56.

After the charging valve 56 is closed, the charging circuit 42determines whether the pressure applied by the second pilot 66 is lessthan the combined pressure applied by the spring 62 and the first pilot64, per step 186. If the pressure applied by the second pilot 66 is notless than the combined pressure applied by the spring 62 and the firstpilot 64, the charging circuit 42 continues to monitor the pressuresuntil such a condition occurs. If the pressure applied by the secondpilot 66 is less than the combined pressure applied by the spring 62 andthe first pilot 64, at step 188, the charging circuit 42 opens thehysteresis valve 76 and the charging valve 56.

The method 170 then repeats steps 176 through 188 during operation ofthe vehicle 10. While the charging circuit 42 continuously charges theaccumulator 114, the steering system 34 also monitors the bypass circuit44 for a loss of steering pressure, per step 190. If there is no loss ofsteering pressure, the charging circuit 42 continues to operate asalready described. However, if there is a loss of steering pressure, atstep 192, the steering system 34 opens the backup steering valve 128 touse the hydraulic fluid from the accumulator 114 for the steeringcontrol unit 46.

It may be appreciated that when the vehicle 10 is turned off, thesolenoid 124 is no longer energized so the discharge valve 116 opens.Thus, the hydraulic fluid stored in the accumulator 114 during operationis drained into the reservoir 36. It should be noted that certain stepsdescribed above may be performed in a different order than described.Further, certain steps that are inherent in a steering system 34 occur,such as operating the pump 38 to pump hydraulic fluid through thesystem.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method for charging an accumulator of a vehicle steering system, the vehicle steering system including a hydraulic pump, the accumulator, an accumulator charging circuit, a bypass circuit, and a steering control unit, the hydraulic _(P)UMP supplying hydraulic fluid to the accumulator charging circuit and the bypass circuit, the method of charging an accumulator comprising: opening a charging valve of the accumulator charging circuit to allow hydraulic fluid to flow from a hydraulic pump to an accumulator; closing a hysteresis valve of the accumulator charging circuit when the hydraulic fluid in the accumulator reaches a first fluid pressure, wherein before the hysteresis valve is closed, hydraulic fluid flowing through the hysteresis valve applies pressure via a first pilot of the charging valve; closing the charging valve after the hysteresis valve is closed, wherein the charging valve is closed when the combined pressure applied by the first pilot and a spring drops below pressure applied by a second pilot of the charging valve; and opening the charging valve to allow hydraulic fluid to flow from the hydraulic pump to the accumulator when the pressure applied by the second pilot of the charging valve decreases below the combined pressure applied by the first pilot and the spring, wherein the accumulator is charged only when the bypass circuit detects a loss of hydraulic fluid pressure.
 2. The method of claim 1, comprising inhibiting hydraulic fluid flow from the accumulator to a hydraulic fluid reservoir when a solenoid of an accumulator drain valve is energized.
 3. The method of claim 2, comprising allowing hydraulic fluid flow from the accumulator to the hydraulic fluid reservoir when the solenoid of the accumulator drain valve is de-energized.
 4. The method of claim 1, comprising comparing a steering hydraulic fluid pressure to a backup steering valve spring pressure and inhibiting hydraulic fluid flow from the accumulator to a steering control unit when the steering hydraulic fluid pressure is greater than the backup steering valve spring pressure.
 5. The method of claim 4, comprising allowing hydraulic fluid flow from the accumulator to the steering control unit when the steering hydraulic fluid pressure is less than the backup steering valve spring pressure.
 6. The method of claim 1, comprising inhibiting hydraulic fluid flow from the accumulator to the charging valve using a check valve.
 7. The method of claim 1, comprising limiting a hydraulic fluid flow rate from the charging valve to the accumulator when the charging valve is open. 